ICSR (Soldering and Reliability) Conference Proceedings


Authors: Chrys Shea, et al.
Company: Cookson Electronics, Cisco Systems, Amkor Technology, Hewlett-Packard, and Jabil Circuit
Date Published: 5/16/2008   Conference: ICSR (Soldering and Reliability)

Abstract: Some Ball Grid Array suppliers are migrating their sphere alloys from SAC305 (3% Ag) or 405 (4% Ag) to alloys with lower silver contents. There are a numerous perceived benefits to this move in terms of cost and performance, but process compatibility and reliability concerns have yet to be addressed.

Process compatibility concerns stem from the fact that the low-silver SAC replacement alloys have higher melting temperatures than SAC305, approximately 227C as compared to 221C. Certain families of electronic assemblies, such as consumer portables, are often heat sensitive and are reflowed in the low end of the established lead-free peak temperature range, typically 230-235C. The small temperature difference between the spheres’ melting temperature and the peak reflow temperature raises questions about the reliability of the solder joints that are formed under this tight thermal margin. These are similar to the concerns raised with the backward compatibility of SAC305/405 spheres with tinlead solder processes. Some of the solutions identified in the lead-free ball/tin-lead paste scenario may apply to the low-silver ball/SAC305 paste combination, but they require review for their applicability with this new set of mixed metals.

A study has been undertaken to characterize the influence of alloy type and reflow parameters on low-silver SAC spheres assembled with backward compatible pastes and profiles. The DOE combines low-silver sphere materials with tin-lead and lead-free solders at different combinations of peak temperature and times above liquidus. Solder joint formation and reliability are assessed to provide a basis for developing practical reflow processing guidelines.

KEY WORDS: Lead-free, low silver, BGA, reflow, mixed metals

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